Tantalum is currently used extensively in the electronics industry, which employs tantalum in the manufacture of highly effective electronic capacitors. Its use is mainly attributed to the strong and stable dielectric properties of the oxide film on the anodized metal. Both wrought thin foils and powders are used to manufacture bulk capacitors. In addition, thin film capacitors for microcircuit applications are formed by anodization of tantalum films, which are normally produced by sputtering. Tantalum is also sputtered in an Ar—N2 ambient to form an ultra thin TaN layer which is used as a diffusion barrier between a Cu layer and a silicon substrate in new generation chips to ensure that the cross section of the interconnects can make use of the high conductivity properties of Cu. It is reported that the microstructure and stoichiometry of the TaN film are, unlike TiN, relatively insensitive to the deposition conditions. Therefore, TaN is considered a much better diffusion barrier than TiN for chip manufacture using copper as metallization material. For these thin film applications in the microelectronics industry, high-purity tantalum sputtering targets are needed.
The typical tantalum target manufacture process includes electron-beam (EB) melting ingot, forging/rolling ingot into billet, surface machining billet, cutting billet into pieces, forging and rolling the pieces into blanks, annealing blanks, final finishing, and bonding to backing plates. The texture in tantalum plate is very dependent on processing mechanisms and temperatures. According to Clark et al. in the publication entitled “Effect of Processing Variables on Texture and Texture Gradients in Tantalum” (Metallurgical Transactions A, September 1991), the texture expected to develop in cold-rolled and annealed body-centered cubic (bcc) metals and alloys consists of orientations centered about the ideal orientations, {001}<110>, {112}<110>, {111}<110>, and {111}<112>. Generally, conventionally processed tantalum is forged or rolled from ingot to final thickness, with only one (1) or no intermediate annealing stages. A final anneal is usually applied to the plate simply to recrystallize the material. The direction of the deformation influences the strengths of resulting annealed textures but generally little attention is given to the resulting distribution of textures. In conventionally processed tantalum, significant texture variation exists in the cross-section of the plate, as described by Clark et al., “Influence of Transverse Rolling on the Microstructural and Texture Development in Pure Tantalum,” Metallurgical Transactions, Vol. 23A, August 1992, p. 2183-2191m; Raabe et al., “Texture and Microstructure of Rolled and Annealed Tantalum,” Materials Science and Technology, Vol. 10, April 1994, p. 299-305; and Michaluk et al., “Methodologies for Determining the Global Texture of Tantalum Plate Using X-ray Diffraction,” Tantalum, The Minerals, Metal & Materials Society, 1996, p. 123-131.
Typically the above mentioned textures exist in stratified bands through the thickness of the rolled plate, or form a gradient of one texture on the surface usually {100}<uvw>, with a gradual transition to a different texture at the centerline of the plate, usually {111}<uvw>. Wright et al., “Effect of Annealing Temperature on the Texture of Rolled Tantalum and Tantalum-10 wt. % Tungsten” (Proceedings of the 2nd International Conference on Tungsten and Refractory Metals, pg 501-508, 1994). Another cause of texture variation through the target thickness is the non-uniformity of the deformation processes used to form the plate. Texture non-uniformity results in variable sputter deposition rates and sputter surface irregularities, which in turn is believed to be a source of micro-arcing.
Micro-arcing is believed to believed to be the principle cause of particle generation and is thus undesirable in the semiconductor industry. FIG. 1 shows the sputter surface of a mixed-texture tantalum target made by conventional processing methods. The sputter surface reveals regions of two different crystallographic textures; dark areas are {100}<uvw>, lighter areas {111}<uvw>. The type of pattern illustrated in FIG. 1 is believed to contribute to sputter film nonuniformities because of the different sputter rates associated with each texture.
FIG. 2 shows severe textural banding in the cross-section of a sputtered tantalum target manufactured according to conventional processes. “Textural banding,” refers to a localized concentration of one texture in the cross section strung out over several grains in a matrix of another texture. In tantalum, it is typically {100}<uvw> textures in a matrix of the more prominent {111}<uvw> textures. For example, a series of grains with the same {100}<uvw> texture in a matrix of {111}<uvw> that are aligned in an elongated manner over several grains is considered a banded textural feature. Using Electron Backscatter Diffraction, EBSD, imaging the texture in small, localized areas can be determined accurately.
In FIG. 2, it can be clearly seen that areas of {100}<uvw> type textures sputter at a greater rate than {111}<uvw> type textures. Thus, any textural non-uniformity at the target surface can produce surface “ridges,” which have an increased likelihood of causing micro-arcing.